The present invention relates to an image display apparatus and its driving method and, particularly, to the image display apparatus driven by a common driving electrode per predetermined number of pixels or per predetermined display region on a display panel and to the driving method thereof.
Conventionally, a plasma display apparatus using a plasma display panel (PDP) for a surface discharge has been commercially available as a flat-type image display apparatus, and has been used as, for example, a display apparatus such as a personal computer and a work station, a flat-type wall-mounted television, and a apparatus for displaying advertisements, information, or others. Also, a flat-type image display unit such as an EL panel has also been used as a display unit for a cellular phone or a personal digital assistant (PDA). These flat-type image display apparatuses such as plasma display apparatuses and EL panels are driven by a common driving electrode with respect to a pixel of one scanning-directional line in a display panel having a plurality of pixels. Note that the present invention is not limited to an image display apparatus driven by the common driving electrode with respect to the pixel of the one scanning-directional line, and may be directed to an image display apparatus driven by the common driving electrode per predetermined number of pixels on a display panel having the plurality of pixels or an image display apparatus driven by the common driving electrode per predetermined display region.
FIG. 1 is a block diagram schematically showing a plasma display apparatus as one example of a conventional image display apparatus and shows one example of a three-electrode surface discharge AC plasma display apparatus. In FIG. 1, a reference numeral “1” denotes an image display apparatus (plasma display apparatus), “2” denotes a display panel (plasma display panel: PDP), “3” denotes an address data driver circuit unit, “4” denotes an X driver circuit unit, “5” denotes a Y driver circuit unit, “6” denotes a scan driver circuit unit, and “7” denotes a control circuit unit.
The plasma display apparatus 1 includes the PDP 2; the X driver circuit unit 4, the Y driver circuit unit 5, the address data driver circuit unit 3, and the scan driver circuit unit 6 for driving each display cell of the PDP 2; and the control circuit unit 7 that controls each of these driver circuit units 3 to 6. The control circuit unit 7 includes, for example, a display data control section 71 to which video signals of three primary colors, R (red), G (green), and B (blue) are supplied from an external apparatus such as a TV tuner or a computer, and a timing generating section 72 to which various synchronization signals (a dot clock signal CLK, a blanking signal XBLANK, a horizontal synchronization signal XHsync, and a vertical synchronization signal XVsync) are supplied. The control circuit unit 7 (display data control section 71 and timing generating section 72) outputs a control signal suitable for each of the driver circuit units 3 to 6 from the above-mentioned video signals (R, G, and B) and various synchronization signals (CLK, XBLANK, XHsync, and XVsync), thereby making a predetermined image display. Note that, for example, for a desired gray-scale display, one field is converted by the display data control section 71 into a combination of a plurality of subfields each having a predetermined weight of luminance.
FIG. 2 is a view for explaining a problem arising in the conventional image display apparatus, and conceptually shows the case where an image in which the entire screen is gray (for example, at a luminance level of 135 out of 256 luminance levels) and only partial regions (P21 and P22) are black (at a luminance level of 0) is displayed.
As shown in FIG. 2, in the conventional image display apparatus (for example, plasma display apparatus), when the image in which the entire screen is at a luminance level of 135 and the only partial regions P21 and P22 are at a luminance level of 0 is displayed, a voltage drop state on its line (display line including pixels corresponding to the regions P21 and P22) is different from that on another line (display line having only the pixels that become at a luminance level of 135), whereby a difference in brightness occurs on the display image and the image quality is degraded.
Specifically, for example, on the lines including the pixels corresponding to the regions P21 and P22 at a luminance level of 0, a display ratio is smaller than that on the other lines having only the pixels that becomes at a luminance level of 135, so that the voltage drop state is also low. As a result, in FIG. 2, on the line including the pixels corresponding to the regions P21 and P22, for example, regions P31, P32, and P33 are brighter than another region (region P1), whereby non-uniformity (luminous difference: difference in brightness) is caused on the display image.
Moreover, since size of the regions P21 and P22 at a luminance level of 0 is changed in directions of arrows (horizontal direction), the brightness of the regions P31, P32, and P33 is also changed. That is, if each size of the regions P21 and P22 is increased, the voltage drop is further decreased, so that the regions P31, P32, and P33 (on the same line) driven by a common electrode together with the regions P21 and P22 become further brighter. Conversely, if each size of the regions P21 and P22 is decreased, the voltage drop is increased (to become close to a voltage drop on the other display lines), so that the regions P31, P32, and P33 driven by the common electrode together with the regions P21 and P22 become darker (the brightness becomes closer to the brightness of another region P1).
This is not only a problem of luminance in a monochrome display image but also a problem of being directly related to non-uniformity of color tone in a color display image. In this specification, the “difference in brightness on display image” has a broad meaning including such color non-uniformity of each color (for example, R, G, and B). Also, in this specification, the “pixel” includes, for example, both of individual cells of R, G, and B on the color display panel and a pixel constituted from one set of R, G, and B.
Note that FIG. 2 shows the case where the common driving electrode (for example, X electrode and Y electrode) is provided per predetermined number of pixels (pixel on one line) in a scanning direction. This common electrode is not limited to an electrode provided per scanning-directional line. If the electrode is provided per predetermined display region, a difference in brightness on a display image occurs per region driven by the common driving electrode.
As described above, in the plasma display apparatus, for example, the difference in brightness (luminous difference) per predetermined number of pixels (pixels on one line) driven by the common electrode occurs essentially due to the voltage drops on the X electrode and the Y electrode caused by a sustain discharge current (sustain current). Generally, in the conventional plasma display apparatus, the difference in brightness between lines has been reduced (resolved) by decreasing a bus impedance and a sustain current themselves.
Also, to prevent a luminous difference between lines depending on a display date amount for each line, there is proposed a scheme (for example, Patent Document 1: Japanese Patent Laid-Open Publication No. 09-068945) of counting the display data amount detected per line and controlling the number of times of the sustain discharges (the number of sustain pulses) per line. In principle, this scheme can be expected to be significantly effective for a luminous difference, flicker, and gray-scale linearity occurring per common electrode. However, in order to achieve sufficient effects, control is required per subfield (SF).